Answer
$5.5Km$
Work Step by Step
In the given scenario
$F_e=F_g$
$\frac{Kqe}{r^2}=mg$
This can be rearranged as:
$r=\sqrt{\frac{Kqe}{mg}}$
We plug in the known values to obtain:
$r=\sqrt{\frac{8.99\times 10^9(0.35\times 10^{-9})(1.6\times 10^{-19})}{(1.637\times 10^{-27})(9.81)}}$
$r=550m= 5.5Km$
